Malignant gliomas, the most common form of central nervous system (CNS) cancers, is currently considered essentially incurable. Among the various malignant gliomas, anaplastic astrocytomas (Grade III) and glioblastoma multiforme (GBM; Grade IV) have an especially poor prognosis due to their aggressive growth and resistance to currently available therapies. The present standard of care for malignant gliomas consists of surgery, ionizing radiation, and chemotherapy. Despite recent advances in medicine, the past 50 years have not seen any significant improvement in prognosis for malignant gliomas. Wen et al. Malignant gliomas in adults. New England J Med. 359: 492-507, 2008. Stupp et al. Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma. New England J Med. 352: 987-996, 2005.
The poor response of tumors, including malignant gliomas, to various types of chemotherapeutic agents are often due to intrinsic drug resistance. Additionally, acquired resistance of initially well-responding tumors and unwanted side effects are other problems that frequently thwart long-term treatment using chemotherapeutic agents. Hence, various analogues of chemotherapeutic agents have been prepared in an effort to overcome these problems. The analogues include novel therapeutic agents which are hybrid molecules of at least two existing therapeutic agents. For example, cisplatin has been conjugated with Pt-(II) complexes with cytotoxic codrugs, or conjugated with bioactive shuttle components such as porphyrins, bile acids, hormones, or modulators that expedite the transmembrane transport or the drug accumulation within the cell. (6-Aminomethylnicotinate) dichloridoplatinum(II) complexes esterified with terpene alcohols were tested on a panel of human tumor cell lines. The terpenyl moieties in these complexes appeared to fulfill a transmembrane shuttle function and increased the rate and extent of the uptake of these conjugates into various tumor cell lines. Schobert et al. Monoterpenes as Drug Shuttles: Cytotoxic (6-minomethylnicotinate) dichloridoplatinum(II) Complexes with Potential To Overcome Cisplatin Resistance. J. Med. Chem. 2007, 50, 1288-1293.
Metastasized cancer, such as breast cancer, that has spread to the brain poses a similarly serious therapeutic challenge as malignant gliomas. This challenge once was a late aspect of disease progression, but increasingly is becoming a first site of disease progression after otherwise successful treatment of primary tumor and metastases outside the cranium. Traditional breast cancer therapeutics, such as paclitaxel or doxorubicin, only reach brain metastases at concentrations that are far lower than needed to be therapeutically active. P. R. Lockman, et al. Heterogeneous blood-tumor barrier permeability determines drug efficacy in experimental brain metastases of breast cancer, Clin Cancer Res 16 (2010) 5664-5678. The most critical barrier to effective entry of chemotherapeutics into the brain is the blood brain barrier (BBB), and very few anticancer drugs are able to overcome this obstacle. E. Fokas, J. P. Steinbach, C. Rodel, Biology of brain metastases and novel targeted therapies: time to translate the research, Biochim Biophys Acta 1835 (2013) 61-75.
Perillyl alcohol (POH), a naturally occurring monoterpene, has been suggested to be an effective agent against a variety of cancers, including CNS cancer, breast cancer, pancreatic cancer, lung cancer, melanomas and colon cancer. Gould, M. Cancer chemoprevention and therapy by monoterpenes. Environ Health Perspect. 1997 June; 105 (Suppl 4): 977-979. Hybrid molecules containing both perillyl alcohol and retinoids were prepared to increase apoptosis-inducing activity. Das et al. Design and synthesis of potential new apoptosis agents: hybrid compounds containing perillyl alcohol and new constrained retinoids. Tetrahedron Letters 2010, 51, 1462-1466.
The alkylating agent temozolomide (TMZ) is able to cross the BBB after oral dosing and has become the chemotherapeutic standard of care for patients with glioblastoma multiforme (GBM). Zhang et al. Temozolomide: mechanisms of action, repair and resistance. Curr Mol Pharmacol 5 (2012) 102-114. TMZ acts as a prodrug. Its mechanism of activation involves hydrolytic opening of its tetrazinone ring, which takes places spontaneously in aqueous solution at 37° C., and does not require the participation of cellular enzymes. The resulting product, the unstable monomethyl MTIC (5-(3-methyltriazen-1-yl)-imidazole-4-carboxamide), reacts with water to liberate AIC (4-amino-5-imidazole-carboxamide) and the highly reactive methyldiazonium cation, which methylates DNA purine residues.
When TMZ was tested for activity against brain metastatic breast cancer in heavily pretreated patients, it revealed mixed outcomes that ranged from “encouraging activity” and “disease control” to “well-tolerated, but no objective responses”. C. Christodoulou et al., Phase II study of temozolomide in heavily pretreated cancer patients with brain metastases, Annals Oncol 12 (2001) 249-254; L. E. Abrey et al., A phase II trial of temozolomide for patients with recurrent or progressive brain metastases, J Neurooncol 53 (2001) 259-265; M. E. Trudeau et al., Temozolomide in metastatic breast cancer (MBC): a phase II trial of the National Cancer Institute of Canada-Clinical Trials Group (NCIC-CTG) Annals Oncol 17 (2006) 952-956; R. Addeo et al. Phase 2 trial of temozolomide using protracted low-dose and whole-brain radiotherapy for nonsmall cell lung cancer and breast cancer patients with brain metastases, Cancer 113 (2008) 2524-2531; S. Siena et al., Dose-dense temozolomide regimen for the treatment of brain metastases from melanoma, breast cancer, or lung cancer not amenable to surgery or radiosurgery: a multicenter phase II study. Annals Oncol 21 (2010) 655-661; R. Addeo et al., Protracted low dose of oral vinorelbine and temozolomide with whole-brain radiotherapy in the treatment for breast cancer patients with brain metastases, Cancer Chemother Pharmacol 70 (2012) 603-609. The underlying basis for these inconsistent results was not investigated, but it is conceivable that these differences may have been due to variable expression levels of O6-methylguanine-DNA methyltransferase (MGMT; also called O6-alkylguanine-DNA alkyltransferase, AGT), a DNA repair enzyme that removes alkyl groups located on the O6-position of guanine A. E. Pegg, Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools, Chem Res Toxicol 24 (2011) 618-639; M. Christmann et al., O(6)-Methylguanine-DNA methyltransferase (MGMT) in normal tissues and tumors: enzyme activity, promoter methylation and immunohistochemistry, Biochim Biophys Acta 1816 (2011) 179-190. Because the primary toxic DNA lesion set by TMZ is alkylation of O6-guanine, high expression levels of MGMT protect tumor cells from the cytotoxic impact of TMZ and provide treatment resistance. J. R. Silber et al., O(6)-methylguanine-DNA methyltransferase in glioma therapy: promise and problems, Biochim Biophys Acta 1826 (2012) 71-82; A. V. Knizhnik et al., Survival and death strategies in glioma cells: autophagy, senescence and apoptosis triggered by a single type of temozolomide-induced DNA damage, PLoS One 8 (2013) e55665. When MGMT expression was investigated in breast cancer metastases to the brain, it was found that over half of the intracranial lesions analyzed were strongly positive for MGMT immunoreactivity. B. Ingold et al., Homogeneous MGMT immunoreactivity correlates with an unmethylated MGMT promoter status in brain metastases of various solid tumors, PLoS One 4 (2009) e4775.
MGMT activity is unusual in that it represents a “suicide” mechanism, whereby acceptance of the alkyl group from DNA irreversibly inactivates the enzyme and leads to its rapid degradation. This feature is exploited by the use of specific MGMT inhibitors, such as O6-benzylguanine (O6-BG), which act as pseudosubstrates. B. Kaina, et al. Targeting O(6)-methylguanine-DNA methyltransferase with specific inhibitors as a strategy in cancer therapy, Cell Mol Life Sci 67 (2010) 3663-3681. Benzylation of MGMT via reaction with O6-BG causes the same structural change in the enzyme as that seen after alkylation following DNA repair, and therefore also leads to rapid degradation of the protein. A. E. Pegg, et al., Use of antibodies to human O6-alkylguanine-DNA alkyltransferase to study the content of this protein in cells treated with O6-benzylguanine or N-methyl-N′-nitro-N-nitrosoguanidine, Carcinogenesis 12 (1991) 1679-1683. Ablation of MGMT activity after treatment of MGMT-positive cells with O6-BG generally increases their sensitivity to killing by TMZ, and this has been well established in numerous in vitro and in vivo tumor models. However, a recent phase-II clinical trial yielded mixed outcomes when O6-BG and TMZ were administered to brain cancer patients with TMZ-resistant tumors: while the addition of the MGMT inhibitor restored TMZ-sensitivity in a fraction (16%) of patients with anaplastic glioma, there was no significant effect (3%) in patients with GBM. J. A. Quinn, et al., Phase II trial of temozolomide plus o6-benzylguanine in adults with recurrent, temozolomide-resistant malignant glioma, J Clin Oncol 27 (2009) 1262-1267. While the underlying reasons for this disappointing outcome remain to be established, the limited response documented in this trial does not generate enthusiasm for the potential study of this drug combination in brain metastatic breast cancer patients.
There is a need to prepare effective therapeutic agents and methods of use thereof in the treatment of cancers such as malignant gliomas and other cancers metastasized in the brain.